The semiconductor processing industry relies on lithography to create patterns on substrates. As the density of features on substrates increases with the progress of Moore's Law, lithographic methods have been challenged to produce repeatable patterns with smaller features over large areas with high throughput.
Standard lithographic methods utilize a mask to cover portions of the substrate to be protected from patterning radiation. In standard photolithography, a light-sensitive photoresist is applied uniformly over the substrate and then exposed to radiation through a pattern-bearing reticle. The resolution of this process is limited by the wavelength of light used, which is typically 248 nm or 193 nm in conventional UV processes. A historical advantage of many photolithography processes was that an entire substrate could be exposed at once, improving throughput. The key disadvantage today is that current photolithography processes struggle to resolve features smaller than about 50 nm in size. For example, some current photolithography processes may create features smaller than about 50 nm in size, but only over a small field of a substrate. Finer resolution over a large field will be needed for future fabrication at dimensions smaller than 50 nm.
Electron beam, or e-beam, lithography is capable of very fine resolution. A substrate is similarly covered with an e-beam sensitive photoresist and then exposed to e-beam radiation. A disadvantage of this technique is that the exposure must be accomplished by scanning the substrate with a beam of electrons. Each spot on the substrate must be lighted with the beam. This takes time, reduces throughput considerably, and introduces problems of uniformity. Hybrid processes involving UV lithography for larger features followed by e-beam lithography for smaller features may improve the result, but such processes are prohibitively expensive, and only effective when there are significant features of larger dimension. As devices become smaller, features with dimension large enough to be resolved by UV lithography become increasingly rare.
In the manufacture of flat panel displays, for example, large substrates up to and exceeding 1220×1400 mm are currently subjected to optical lithography processes such as proximity printing, step and repeat lithography, multi-lens scanning, and mirror projection. Proximity printing, multi-lens scanning, and mirror projection typically use very large area masks comparable in size to the substrates being processed. These masks may cost up to $1 million each. Moreover, as the masks grow larger, they must be made thicker and heavier to survive handling during the process, and to minimize physical distortion of the mask. In some cases, such physical distortion can only be overcome by using complicated and expensive optics.
Pattern transfer by physical contact is a promising technique for patterning substrates, including large area substrates, at dimensions less than 50 nm and extending up through many tens of microns. A pattern is developed in a template, and then physically applied to the substrate. The template serves as a pattern transfer medium for the patterning process. Efficient, high-throughput methods are still needed, however, to fully implement physical contact lithography for mass production of next generation devices.